1. Field of the Invention
The present invention refers to a pulsator according to the preamble of claim 1.
Today, such pulsators are used in milking machines for alternate pulsation of two pairs of teatcups between a suction phase, i.e. usually a sub-atmospheric pressure, and a pressure phase, i.e. usually atmospheric pressure. During alternate pulsation, the pulsation cycle, i.e. the sum of the suction time and the pressure time, for one of the pairs of teatcups is displaced in relation to the pulsation cycle for the other pair. This means that one of the teatcup pairs is essentially in the suction phase and the other of the pairs essentially in the pressure phase. It is important that the suction phases of both the teatcup pairs have the same duration in order to avoid an uneven milking of the teats. This difference between the duration of the suction phases which is provided by a pulsator is called limping.
2. Description of the Prior Art
SE-B-344 531 discloses a pulsator of the type initially defined and intended for a milking machine. The adjustment of the second circuit is performed by this known pulsator by means of a change-over mechanism, comprising a change-over arm provided on the valve element, a change-over spring and a valve slide. The change-over arm is pivotably attached to a housing of the pulsator and is forcibly pivoted back and forth about a pivot point by means of the movements back and forth of the valve element. The valve slide has two curved surfaces which are intended to slide in turn against a corresponding curved surface of the change-over arm. These surfaces co-operate for controlling the time moments for the change-over of the valve slide from one end position to another. One problem with this known pulsator is that the change-over mechanism comprises many moveable, mechanical construction parts which have to be manufactured with high precision, which makes the production of the pulsator expensive. Due to the many parts and the friction which arises between these parts, there is also a slowness of the change-over mechanism, which results in a mechanical work to be overcome. In addition, the level of the slowness is difficult to determine and is changed with the wear of the parts of the change-over mechanism. Moreover, due to the wear, regular service of the pulsator is required in order to enable the maintenance of a desired pulsation accuracy. Furthermore, the limping of this known pulsator is at least not easily adjustable.